Vue.js

What is Vue.js?

Vue (pronounced /vjuː/, like view) is a progressive framework for building user interfaces. Unlike other monolithic frameworks, Vue is designed from the ground up to be incrementally adoptable. The core library is focused on the view layer only, and is easy to pick up and integrate with other libraries or existing projects. On the other hand, Vue is also perfectly capable of powering sophisticated Single-Page Applications when used in combination with modern tooling and supporting libraries.

If you’d like to learn more about Vue before diving in, we created a video walking through the core principles and a sample project.

Getting Started

The official guide assumes intermediate level knowledge of HTML, CSS, and JavaScript. If you are totally new to frontend development, it might not be the best idea to jump right into a framework as your first step - grasp the basics then come back! Prior experience with other frameworks helps, but is not required.

The Installation page provides more options of installing Vue. Note: We do not recommend that beginners start with vue-cli, especially if you are not yet familiar with Node.js-based build tools.

Declarative Rendering

At the core of Vue.js is a system that enables us to declaratively render data to the DOM using straightforward template syntax:

<div id="app">
{{ message }}
</div>

var app = new Vue({
el: '#app',
data: {
message: 'Hello Vue!'
}
})

We have already created our very first Vue app! This looks pretty similar to rendering a string template, but Vue has done a lot of work under the hood. The data and the DOM are now linked, and everything is now reactive. How do we know? Open your browser’s JavaScript console (right now, on this page) and set app.message to a different value. You should see the rendered example above update accordingly.

In addition to text interpolation, we can also bind element attributes like this:

Here we are encountering something new. The v-bind attribute you are seeing is called a directive. Directives are prefixed with v- to indicate that they are special attributes provided by Vue, and as you may have guessed, they apply special reactive behavior to the rendered DOM. Here, it is basically saying “keep this element’s title attribute up-to-date with the message property on the Vue instance.”

If you open up your JavaScript console again and enter app2.message = 'some new message', you’ll once again see that the bound HTML - in this case the title attribute - has been updated.

Conditionals and Loops

It’s easy to toggle the presence of an element, too:

<div id="app-3">
<span v-if="seen">Now you see me</span>
</div>

var app3 = new Vue({
el: '#app-3',
data: {
seen: true
}
})

Go ahead and enter app3.seen = false in the console. You should see the message disappear.

This example demonstrates that we can bind data to not only text and attributes, but also the structure of the DOM. Moreover, Vue also provides a powerful transition effect system that can automatically apply transition effects when elements are inserted/updated/removed by Vue.

There are quite a few other directives, each with its own special functionality. For example, the v-for directive can be used for displaying a list of items using the data from an Array:

Composing with Components

The component system is another important concept in Vue, because it’s an abstraction that allows us to build large-scale applications composed of small, self-contained, and often reusable components. If we think about it, almost any type of application interface can be abstracted into a tree of components:

In Vue, a component is essentially a Vue instance with pre-defined options. Registering a component in Vue is straightforward:

// Define a new component called todo-item
Vue.component('todo-item', {
template: '<li>This is a todo</li>'
})

But this would render the same text for every todo, which is not super interesting. We should be able to pass data from the parent scope into child components. Let’s modify the component definition to make it accept a prop:

Vue.component('todo-item', {
// The todo-item component now accepts a
// "prop", which is like a custom attribute.
// This prop is called todo.
props: ['todo'],
template: '<li>{{ todo.text }}</li>'
})

Now we can pass the todo into each repeated component using v-bind:

<div id="app-7">
<ol>
<!--
Now we provide each todo-item with the todo object
it's representing, so that its content can be dynamic.
We also need to provide each component with a "key",
which will be explained later.
-->
<todo-item
v-for="item in groceryList"
v-bind:todo="item"
v-bind:key="item.id">
</todo-item>
</ol>
</div>

This is a contrived example, but we have managed to separate our app into two smaller units, and the child is reasonably well-decoupled from the parent via the props interface. We can now further improve our <todo-item> component with more complex template and logic without affecting the parent app.

In a large application, it is necessary to divide the whole app into components to make development manageable. We will talk a lot more about components later in the guide, but here’s an (imaginary) example of what an app’s template might look like with components:

Relation to Custom Elements

You may have noticed that Vue components are very similar to Custom Elements, which are part of the Web Components Spec. That’s because Vue’s component syntax is loosely modeled after the spec. For example, Vue components implement the Slot API and the is special attribute. However, there are a few key differences:

The Web Components Spec is still in draft status, and is not natively implemented in every browser. In comparison, Vue components don’t require any polyfills and work consistently in all supported browsers (IE9 and above). When needed, Vue components can also be wrapped inside a native custom element.

Vue components provide important features that are not available in plain custom elements, most notably cross-component data flow, custom event communication and build tool integrations.

Ready for More?

We’ve briefly introduced the most basic features of Vue.js core - the rest of this guide will cover them and other advanced features with much finer details, so make sure to read through it all!